digging out this one
We have the same issue with Imgur, embed not working.
In the composer preview, following error message appears:
Sorry, we were unable to generate a preview for this web page, because the web server returned an error code of 429. Instead of a preview, only a link will appear in your post.
example link: How To: Quickstop - FAQs & How-tos - pev.dev (scroll to bottom of first post)